The present invention relates to antenna systems, and in particular to an antenna system which dynamically selects the output with the greatest radio frequency power from two or more fixed beam array antennas focused in the near field.
The focused transfer of electromagnetic energy over a relatively short distance in free space is desirable in systems where mechanical connectivity between data transfer points is not feasible due to physical constraints of the system. For example, in the locomotive environment, it is desirable to transmit data from a video camera mounted at a station platform to a receiving unit located on a locomotive car, which may be moving in relation to the platform or which may be stationary. In the fixed beam array antennas of the present invention, a transmitting antenna in the form of a leaky coaxial cable is disposed linearly along the train track, and the video data to be transmitted to the receiving unit on the stationary or moving car is modulated on an electromagnetic wave carrier operating in the microwave frequency region. It is therefore required to have a receiving antenna located relatively near but not in contact with the leaky coaxial cable. The fixed beam array antenna may be in motion relative to the leaky coaxial cable while the system transmits data across the link established thereby.
Again, due to the physical constraints of the system, the electromagnetic signal emanating from the leaky coaxial cable is of relatively low power, typically in the range of -90 dBm. It is thus critical to utilize an antenna with a gain as high as practicable while keeping the size of the antenna relatively small for physical as well as economic considerations.
It is therefore an object of the present invention to provide an antenna which is useful for receiving focused electromagnetic energy from a point in its near field that is inexpensive, small and practical to manufacture.
Phased array antennas are used in the art in order to maximize gain when required by coherently adding signals incident on each constituent element of the array. Moreover, linear phased array antennas are well known in the art and are useful in forming a beam in the far field of the antenna and electronically scanning the beam by varying the phase shift across the array in a linear fashion. For example, U.S. Pat. No. 3,999,182 teaches such an antenna useful in aircraft landing systems operating in the microwave frequency range.
Linearly phase shifted array antennas are not useful, however, in operating in the near field, and in particular, are not useful in obtaining a fixed beam focused signal with relatively high gain for signals of relatively low power, since as array aperture increases in size (which provides an increase in antenna gain), the distance at which the array is focused increases. This limits the efficacy of high gain antennas at shorter distances.
With respect to applications such as the locomotive environment described above, lateral and vertical displacement of the locomotive as it traverses a set of train tracks will generally result in variations in the distance between the leaky coaxial cable disposed linearly along the train track and the antenna mounted on the locomotive. Since the focal point of the antenna is fixed, variations in this distance may lead to poor reception of the desired signal.
It is therefore an object of the present invention to provide an antenna system which is tolerant of variations in the distance between the antenna and the transmitting unit in its near field.